Each of DeSantis et al., U.S. Pat. No. 4,328,501, and Reggia, U.S. Pat. No. 3,838,429, discloses a small, top-loaded antenna having a top load formed as a metal conducting plate extending generally parallel to a ground plane through which a monopole radiating element extends. In each instance, the top load is capacitively coupled to the monopole radiating element.
In the DeSantis et al. antenna, a center conductor of a coaxial cable extending through the radiating element is electrically connected to the top load by an inductor shunted by a resistor. The network including the shunt resistor and inductor makes the antenna broadband but lossy. The DeSantis et al. antenna thus has low radiation efficiency, requiring relatively high amplitude sources to drive the antenna for long range applications. Since short antennas are generally used on mobile structures having limited power, such as aircraft, the low efficiency DeSantis et al. antenna is not suitable for long range applications.
The Reggia antenna is characterized as including a spiral as part of the top load for a short vertical monopole radiator. The spiral functions as an open circuit transmission line. Currents in two sides of the transmission line are oppositely directed, tending to cancel the effect of each other in a distant field of the antenna. Thereby, only the short vertical monopole radiator contributes to the distant field radiation. However, the printed circuit spiral is a narrow band device capable of being varied only by movement of a mechanical element, such as probe transversing the spiral. Hence, the Reggia structure cannot be rapidly tuned over a wide bandwidth and operate with high efficiency.
One type of variable inductor that has been used for tuning includes plural series-connected inductive elements having values related to each other in a binary manner, such that, for example, the values of the inductive elements are L, L/2, L/4, L/8, L/16, L/32, etc. Different ones of the inductive elements are selectively short-circuited, to control the value of the inductor. Short-circuiting is accomplished in response to a binary signal having multiple bits, one of which is provided for each of the inductive elements. Each inductive element is short-circuited by connecting opposite terminals of the particular element to a pair of PIN diodes having like electrodes connected to each other and to a binary bit source controlling forward and reverse biasing of the diodes. A source for each binary bit is connected to the diodes associated with the particular inductive element via a separate radio frequency choke, designed so that RF currents flowing through the inductive elements are decoupled from the binary bit source. A return path to the binary bit sources is provided by a further RF choke.
A problem with this prior art variable inductor is that multiple RF chokes are selectively connected in parallel between the several inductive elements and several of the binary bit sources. The parallel connections of the RF chokes have a tendency to draw RF current from the inductor and thereby to affect adversely the performance of the inductor for RF purposes. As the number of bits in a binary inductor bank increases, loading by the RF chokes becomes an increasing problem tending to degrade the quality factor (Q) and alter the value of the individual inductors comprising the bank, causing deviation from the desired binary relationship. Hence, there are disadvantages in tuning the prior art short monopole radiating elements utilizing conventional controllers for the value of a variable inductor.
It is, accordingly, an object of the present invention to provide a new and improved structure for rapidly tuning an efficient short, top-loaded monopole antenna over a relatively wide frequency range.
Another object of the invention is to provide a new and improved variable inductor, particularly adapted for use in connection with efficient, tunable, short, top-loaded monopole antennas.
Still another object of the invention is to provide a new and improved variable inductor including plural series-connected inductive elements selectively short-circuited in response to control signals coupled to the inductive elements in such a manner as to minimize interference with the performance of the inductor in handling RF currents.
Another object of the present invention is to provide a new and improved RF inductor controlled by a binary bit control source so that RF currents flowing through the inductor do not interact with the control source, so the inductor has a high Q, repeatability in changes of inductive value and can operate over a wide frequency band in a uniform manner.